6 a double-concave, and Fig.

7 a concavo-convex or meniscus.

By these it is seen that a double-convex lens tends to condense the rays of light to a focus, a double-concave to scatter them, and a concavo-convex combines both powers.
If parallel rays of light fall upon a double-convex lens, D D, Fig.

8, they will be refracted (excepting such as pass directly through the centre) to a point termed the principal focus.
[Illustration: Fig.

8a (amdg_8a.gif)] The lines A B C represent parallel rays which pass through the lens, D D, and meet at F; this point being the principal focus, its distance from the lens is called the focal length.